I know what you mean. I'm a Firefox lover too. The reason why it spawns IE is because LR is set to use the IE browser within LR, so whatever pops up from within LR will naturally be IE. I do agree IE is a big hog on resources. Will have a chat with the programmers and see if we can let it use whatever is the system's default browser instead of forcing it to use IE.
BTW, have you checked out Google's new browser Chrome - it's awesome, even faster than Firefox!